{3\50 


HAWAII  AGRICULTURAL  EXPERIMENT  STATION 

HONOLULU,  HAW  AH 

Under  the  supervision  of  the 
UNITED  STATES  DEPARTMENT  OF  AGRICULTURE 


BULLETIN  No.  50 


THE  SWEET  POTATO  IN  HAWAII 


BY 


H.  L.  CHUNG,  Specialist  in  Tropical  Agronomy 


•  V.  OF  FL  LIB 
■DOCUMENTS  np°PT 


Issued  October.  1923         .-»«   M 


OSiTORY 


WASHINGTON 
GOVERNMENT  PRINTING  OFFICE 

1923 


HAWAII  AGRICULTURAL  EXPERIMENT  STATION 

HONOLULU,  HAWAII 

Under  the  supervision  of  the 
UNITED  STATES  DEPARTMENT  OF  AGRICULTURE 


BULLETIN  No.  50 


THE  SWEET  POTATO  IN  HAWAII 

BY 
H.  L.  CHUNG,  Specialist  in  Tropical  Agronomy 


Issued  October,  1923 


WASHINGTON 
GOVERNMENT  PRINTING  OFFICE 

1923 


HAWAII  AGRICULTURAL  EXPERIMENT  STATION,  HONOLULU. 

[Under  the  supervision  of  the  Office  of  Experiment  Stations,  United  States  Department  of"  Agricult  im*  J 

E.  W.  Allen,  'Chief,  Office  of  Experiment  Stations. 

Walter  H.  Evans,  Chief,  Division  of  Insular  Stations,  Office  of  Experiment  Stations, 


STAFF. 


,T.  M.  Westgate,  Agronomist  in  Charge. 

H.  L.  Chung,  Specialist  in  Tropical  Agronomy. 

W.  T.  Pope,  Horticulturist . 

J.  C.  Ripperton,  Chemist. 

R.  A.  Goff,  In  Charge  of  Glenwood  Substation  and  Extension  Agent  for  Island  of  Hawaii , 

Nellie  A.  Russell,  Collaborator  in  Home  Economics. 

Mabel  Green,  Boys'  and  Girls'  Club  Leader. 


THE  SWEET  POTATO  IN  HAWAII. 

By  H.  L.  Chung,  Specialist  in  Tropical  Agronomy. 


CONTENTS. 


Introduction 1 

Botanical  relationship 2 

Cultivation  in  ancient  times 2 

Place  in  the  Hawaiian  cropping  system 2 

Adaptation  to  Hawaii 2 

Soil 3 

Methods  of  culture 4 

Grading 9 

Storing 10 


Cost  of  production 10 

InsBct  enemies  and  methods  of  control 11 

Fungus  diseases  and  means  of  controlling 

them 13 

Varieties  of  sweet  potatoes  in  Hawaii 15 

Composition  of  the  sweet  potato 15 

Recipes 16 

Sweet  potatoes  as  feed  for  farm  animals 19 

Summary 19 


INTRODUCTION.1 

It  can  not  be  definitely  determined  when  the  sweet  potato  (Ipornoea 
batatas)  was  first  cultivated  in  Hawaii,  but  it  is  thought  that  the 
crop  has  been  under  cultivation  on  the  island  since  about  500  A.  D.,2 
because  the  taro,  which  is  a  companion  crop  of  the  sweet  potato,  has, 
from  the  earliest  days,  constituted  the  staple  food  crop  of  the  natives. 
Captain  Cook  records  the  finding  in  1778  of  specimens  of  taro  of 
large  size,  and  sweet  potatoes,  weighing  12  to  14  pounds,  in  the 
Hawaiian  Islands,  then  known  as  the  Sandwich  Islands. 

The  sweet  potato  first  became  of  commercial  importance  to  the 
Islands  in  about  1849.  It  ranked  tenth  in  value  of  the  agricultural 
crops  in  1919,  having  been  reintroduced  as  an  emergency  crop. 
The  World  War  was  an  efficient  factor  in  emphasizing  the  need  of 
producing  locally  grown  food  crops  to  make  the  island  independent 
of  imported  foodstuffs;  and  during  this  period  the  sweet  potato  was 
used  in  place  of  potatoes,  occasionally  instead  of  barley  and  oats 
for  farm  animals  and  for  wheat  and  corn  for  poultry,  and  as  a  partial 
substitute  for  wheat  flour  in  the  making  of  bread  and  pastries. 
The  crop  is  no  longer  exported,  owing  to  rigorous  quarantine  regu- 
lations. 

Today  the  sweet  potato  is  found  growing  in  back  yards  and  school 
gardens  and  on  areas  covering  upward  of  50  acres  on  large  ranches 
throughout  the  islands.  Fully  350  acres,  located  principally  on 
the  islands  of  Hawaii  and  Maui,  are  now  under  cultivation.  This 
crop  will  undoubtedly  play  an  important  role  in  the  further  de- 
velopment of  the  diversified  agricultural  industries  of  the  islands, 
more  especially  since  it  affords  large  returns  in  money  from  small 
areas. 

'  The  writer  desires  to  express  his  appreciation  to  those  in  charge  of  the  Bishop  Museum,  Honolulu, 
for  their  courtesy  in  placing  at  his  disposal  information  relative  to  ancient  methods  of  cultivating  the  sweet 
potato  in  Hawaii,  and  also  to  F.  G.  Krauss,  former  superintendent  of  the  Haiku  substation,  who  very 
kindly  contributed  data  obtained  at  that  substation. 

J  Alexander,  W.  D.    A  Brief  History  of  the  Hawaiian  People,  p.  19. 

1 


2  BULLETIN   50,   HAWAII   EXPERIMENT   STATION. 

BOTANICAL  RELATIONSHIP. 

The  sweet  potato  belongs  to  the  morning-glory  family  (Convolvu- 
lacese)  and  is  known  botanically  as  Ipomcea  batatas.  It  is  probably 
indigenous  to  the  West  Indies  or  Central  America.  In  the  Tropics 
this  plant  is  a  perennial  and  produces  long  trailing  stems.  Except 
in  case  of  a  few  varieties,  the  sweet  potato  blooms  profusely  in 
Hawaii  from  November  to  April.  The  shape  of  the  leaves  and  the 
color  of  the  skin  and  flesh  differ  with  the  variety.  The  skin  ranges 
from  white  to  dark  brown  in  color,  and  the  flesh  varies  from  white 
to  pumpkin  and  dark  purple. 

CULTIVATION  IN  ANCIENT  TIMES. 

The  sweet  potato  was  grown  with  care  by  the  ancient  Hawaiians. 
The  vines  for  planting  were  not  gathered  at  random,  but  with  a 
discrimination  which  showed  that  the  native  agriculturists  ap- 
preciated the  importance  of  selection.  Vines  of  individual  plants 
bearing  roots  in  large  quantities  were  selected  for  further  planting. 

PLACE  IN  THE  HAWAIIAN  CROPPING  SYSTEM. 

From  ancient  times  the  native  Hawaiian  farmer  has  realized  the 
advisability  of  alternating  his  sweet  potatoes  with  other  cultivated 
crops  to  improve  the  physical  condition  of  the  soil.  Observation 
taught  him  that  land  which  was  allowed  to  rest  for  a  year  or  so  after 
having  produced  a  sweet-potato  crop  yielded  a  better  and  heavier 
crop  than  did  ground  which  was  kept  continuously  in  one  crop. 
Weeds  and  native  grasses  were  therefore  allowed  to  grow  for  a  reason- 
able length  of  time  before  the  land  was  replanted  to  sweet  potatoes. 

The  rice  planters  of  to-day  use  the  sweet  potato  and  other  cultivated 
crops  in  their  system  of  rotation.  As  soon  as  the  last  annual  grain 
crop  has  been  harvested,  the  coarse  straw  is  burned  and  the  stubble 
is  turned  under  and  harrowed.  The  land  is  then  given  a  heavy  appli- 
cation of  manure  and  is  immediately  used  for  some  quick-growing 
vegetable,  such  as  lettuce,  spinach,  beets,  radish,  or  mustard  cabbage, 
followed  by  sweet  potatoes. 

The  following  cropping  systems  are  recommended  for  use  in  con- 
nection with  sweet-potato  cultivation  in  Hawaii: 

Two-year  rotation. — Corn,  cowpeas,  sweet  potatoes,  peanuts;  or 
sweet  potatoes,  beans,  buckwheat,  cowpeas,  and  corn. 

Three-year  rotation. — Corn,  peanuts,  grass  or  sorghums,  cowpeas, 
and  sweet  potatoes. 

Four-year  rotation. — Corn,  mungo  beans,  sorghum,  alfalfa,  and 
sweet  potatoes. 

Five-year  rotation. — Corn,  peanuts,  sorghum,  pigeon  peas,  sweet 
potatoes;  or  cassava,  cowpeas,  peanuts,  sweet  potatoes,  and  alfalfa. 

ADAPTATION  TO  HAWAII. 

Hawaii,  being  situated  well  within  the  Tropics,  is  naturally  adapted 
to  the  growing  of  sweet  potatoes.  The  situation  is  so  favorable,  in 
fact,  that  the  plant  produces  immense  roots,  even  when  it  is  grown 
in  the  pockets  of  volcanic  rocks  where  there  is  little  soil.  The  sweet 
potato  can  be  grown  on  all  the  areas  of  Hawaii  except  in  rocky 


THE   SWEET  POTATO   IN   HAWAII.  3 

regions  where  there  is  no  soil,  in  locations  where  there  is  an  insuffi- 
cient amount  of  rainfall,  or  on  high  elevations  which  are  exposed  to 
the  wind. 

The  plant  thrives  from  sea  level  to  an  elevation  of  2,000  feet. 
Growth  can  be  maintained  at  a  still  higher  altitude,  provided  the 
location  is  sheltered  from  strong  winds,  which  are  decidedly  harmful 
to  the  plants.  When  grown  at  an  altitude  higher  than  1,500  feet, 
the  period  of  maturity  of  a  given  variety  varies  with  the  altitude; 
that  is,  the  higher  the  elevation,  the  longer  will  be  the  period  required 
to  mature  the  crop,  owing  to  lower  temperatures. 

To  make  its  best  development,  the  crop  needs  moderate  rainfall, 
an  abundance  of  sunshine,  and  warm  nights  from  the  time  of  plant- 
ing until  the  vines  produce  vigorous  axillary  buds. 

SOIL. 

To  produce  its  maximum  yield,  the  sweet-potato  crop  should  be 
grown  in  a  well-drained,  moderately  fertile,  loose  sandy  soil.  The 
following  hints  may  be  of  value  to  planters  living  in  regions  where 
there  is  no  soil  of  this  kind. 

Heavy  clay  soil. — When  the  sweet  potato  is  grown  in  heavy  clay 
soil,  such  as  local  taro  soil,  it  develops  a  dwarfed  and  sickly  vine 
growth  and  coarse  roots  which  are  likely  to  be  unmarketable,  owing 
to  their  irregular  shape.  (PI.  I.)  Clayey  soil  renders  aeration  impos- 
sible, since  it  is  extremely  sticky  during  wet  periods  and  closely  com- 
pacted during  the  hot  summer  months,  when  it  dries  out  in  hard 
lumps. 

To  improve  the  physical  condition  of  such  soil,  leguminous  crops, 
such  as  cowpeas  and  velvet  beans,  should  be  grown  and  plowed 
under  as  green  manure.  The  plowing  under  of  leguminous  crops  adds 
large  quantities  of  nitrogen  to  the  soil.  Preparatory  to  the  planting 
of  a  second  leguminous  crop,  and  before  harrowing  is  done,  other 
organic  matter,  such  as  rotted  stable  manure,  rice  hulls,  or  chaff, 
should  be  broadcasted  on  the  plowed  field.  Rice  hulls  can  be  obtained 
from  any  rice-milling  establishment  in  Hawaii  for  the  asking  and 
cartage.  The  incorporation  of  organic  matter  in  the  clayey  soil 
loosens  and  mellows  the  soil  and  enables  it  to  retain  sufficient  moisture 
for  plant  growth. 

Soils  rich  in  humus. — The  vines  make  luxuriant  growth  at  the 
expense  of  the  roots  when  the  crop  is  grown  on  land  containing  very 
large  quantities  of  humus.  Such  land  should  not  be  planted  to  sweet 
potatoes  for  six  months  following  the  successive  planting  of  crops 
having  edible  foliage,  such  as  green  mustard,  white  mustard,  and 
Chinese  cabbage. 

Soils  in  seashore  areas. — The  sweet  potato  can  be  grown  very 
successfully  on  areas  not  far  from  the  seashore  and  on  soil  containing 
a  large  quantity  of  sand.  Flat  cultivation  rather  than  ridge  planting 
should  be  practiced  on  such  land,  and  organic  matter  and  commercial 
fertilizers  should  be  incorporated  with  the  soil  from  time  to  time 
to  render  it  productive.  Only  those  varieties  of  sweet  potatoes 
which  have  already  been  acclimated  and  are  adapted  to  seashore 
conditions  should  be  planted  on  sandy  soil,  otherwise  the  roots  will 
contain  a  high  percentage  of  salt.  Experiments  conducted  at  the 
experiment  station  on  land  near  the  seashore  showed  that  the  roots 


4  BULLETIN   50,   HAWAII   EXPERIMENT   STATION. 

of  four  varieties  of  sweet  potatoes  contained  from   four   to  seven 
times  as  much  salt  as  did  those  grown  at  the  central  station. 

SOIL  MOISTURE. 

In  Hawaii,  where  there  is  an  uneven  distribution  of  rainfall  as 
well  as  great  diversity  of  soil  types,  the  moisture  content  of  the 
different  soils  varies  considerably.  Some  of  the  soils  are  so  porous 
that  the  water  filters  through  the  surface  instead  of  being  absorbed, 
and  washes  away  much  vegetable  matter.  Such  soils  may  be  im- 
proved in  water-holding  capacity  by  green  manuring.  Other  soils 
are  too  retentive  of  moisture  and  for  this  reason  are  detrimental  to 
the  crop.  The  sweet-potato  crop  should  not  be  planted  until  it  is 
known  what  the  soil  and  rainfall  conditions  are.3 

Virgin  lands,  or  fields  that  have  lain  fallow  for  some  time,  should 
be  plowed  and  all  volunteer  growth  turned  under  to  insure  excellent 
soil  conditions  for  the  development  of  the  roots  of  the  new  crop.  If 
the  soil  contains  a  suitable  amount  of  moisture  the  roots  will  develop 
near  the  surface,  but  if  it  is  dry  they  will  grow  downward  before 
enlarging.     Roots  growing  in  a  dry  soil  are  difficult  to  harvest. 

In  localities  where  the  fields  are  flooded  after  a  heavy  rain  and  the 
water  remains  standing  in  the  furrows  for  several  days,  not  only  are 
the  fertilizers  leached  out  of  the  ridges,  but  the  growth  of  vines  is 
stimulated  at  the  expense  of  the  roots.  All  the  furrows  should 
therefore  lead  into  a  channel  at  the  lowest  part  of  the  field.  After 
the  standing  water  has  been  removed  from  the  furrows,  the  ridges 
should  be  examined.  On  those  which  are  saturated  with  water  the 
vines  should  be  thrown  to  one  side,  so  that  the  wind  and  sun  will 
hasten  evaporation.  After  about  four  days,  when  these  ridges  have 
returned  to  normal  conditions,  the  vines  should  be  thrown  to  the 
opposite  side,  so  that  the  newly  exposed  part  will  dry  out.  (PL 
II,  fig.  1.) 

METHODS  OF  CULTURE. 

PROPAGATING  MATERIAL. 

Sweet  potatoes  are  propagated  either  from  vine  cuttings  or  from 
slips  resulting  from  the  sprouting  of  shoots  from  the  root.  In 
Hawaii  terminal  cuttings  are  used  almost  exclusively,  because  they 
have  the  ability  to  make  quick  growth  and  the  advantage  of  being 
practically  free  from  insect  pests.  Old  stem  cuttings  are  likely  to 
carry  the  eggs  or  larvae  of  the  stem  borer  and  are  often  the  means  of 
infesting  new  fields  with  the  pest.  So  far  as  resulting  yield  is  con- 
cerned, no  appreciable  difference  has  been  found  between  the  ter- 
minal and  old  stem  cuttings. 

In  Hawaii,  where  propagating  material  can  be  readily  taken  from 
the  vine  at  any  time  of  the  year,  the  roots  of  the  sweet  potato  are 
not  usually  bedded  in  the  greenhouse  in  spring  for  slips.  It  is  only 
when  a  variety  shows  signs  of  degeneracy,  in  the  form  of  unproduc- 
tiveness, that  the  sweet-potato  roots  are  sprouted.  The  terminal 
cuttings  are  removed  in  lengths  of  from  12  to  16  inches,  and  the  large 
leaves  and  their  petioles  are  stripped  from  them.  The  cuttings 
should  be  moistened  and  protected  from  drying  when  they  are  not 
to  be  planted  immediately  after  their  removal  from  the  vine.     Fresh- 

*  Planters  may  obtain  a  report  on  their  soils  by  consulting  the  experiment  station. 


THE   SWEET  POTATO   IN   HAWAII. 


cut  vines  which  are  planted  in  beds  without  irrigation  make  quicker 
growth  than  do  partly  shriveled  cuttings  which  are  several  days  old. 
Slips  that  are  not  to  be  planted  immediately  should  be  covered  with 
burlap  bags  saturated  with  water. 


PREPARATION  OF  THE  LAND. 

The  sweet-potato  crop,  like  most  other  vegetable  crops,  gives  best 
results  when  it  is  planted  on  well-prepared  land,  especially  on  land 
that  has  been  planted  with  some  leguminous  crop  the  year  preceding. 
The  area  should  first  be  thoroughly  cleared  of  all  coarse  material  and 
then  plowed  fairly  deep,  the  sod  and  debris  being  turned  under. 
The  depth  of  plowing  is  an  important  factor  in  the  preparation  of 
land  and  exerts  considerable  influence  upon  the  character  of  the  roots. 
The  depth  used  for  corn  will  do  for  sweet  potatoes.  The  fertilizer, 
if  any  is  to  be  applied,  should  then  be  spread  broadcast,  and  the  field 
harrowed  three  or  four  times  to  make  the  soil  sufficiently  mellow. 
Later  a  plank  drag  may  be  used  to  smooth  the  surface. 

A  spading  fork  is  recommended  for  use  in  small  areas.  This  im- 
plement is  very  efficient  for  soils  that  are  not  compact  and  hard,  as 
the  prongs  strike  well  into  the  ground.  Small  areas  which  are 
covered  with  low-growing  weeds  and  other  vegetation  may  be  turned 
under  with  it.  Fertilizers  should  then  be  applied  and  a  rake  used  to 
level  the  area  and  to  mix  the  fertilizer  with  the  soil.  Planting,  either 
by  the  level  or  ridge  method,  may  begin  a  week  later,  when  the 
soil  has  had  ample  time  to  settle. 

PLANTING. 

Three  methods  of  planting  are  practiced  in  Hawaii,  namely,  (1) 
ridge  planting,  (2)  flat  planting,  and  (3)  individual  mound  or  hill 
planting. 

Ridge  planting. — This  method  is  commonly  practiced  by  the 
sweet-potato  growers  of  the  islands,  and  is  of  advantage  in  that  it 
provides  drainage,  insures  aeration,  and  puts  the  soil  in  good  physi- 
cal condition  for  the  best  development  of  the  roots.  The  ridges  are 
constructed  of  loose  earth  and  vegetable  matter  and  stand  from  12 
to  16  inches  high.  They  are  spaced  about  3  or  4  feet  apart  and  are 
planted  with  1  or  2  cuttings  set  1J  and  2  feet  apart  in  the  row.  (PL 
II,  %.  2.) 

Soils  which  are  located  near  the  seashore  should  not  be  ridged,  be- 
cause ridging  tends  to  increase  the  surface  area  and  to  hasten  evapora- 
tion of  soil  moisture. 

Flat  planting. — This  method  of  planting  is  also  extensively  prac- 
ticed, but  more  especially  on  very  sandy  loam  or  sandy  soil.  In 
flat  planting  the  surface  of  the  soil  is  made  flat  or  level  before  planting 
is  done. 

Mound  or  Mil  planting. — The  method  of  setting  plants  in  hills  or 
elevations  of  earth  (PI.  Ill,  fig.  1)  has  been  handed  down  from 
primitive  times  and  has  an  advantage  over  the  other  two  methods, 
so  far  as  the  native  Hawaiians  are  concerned,  in  that  it  permits 
of  the  mound's  being  broken  down  and  the  roots  exposed  at  once  with- 
out great  effort.  The  natives  make  it  a  rule  to  harvest  only  enough 
roots  to  cover  the  needs  of  the  day.  The  mounds  are  from  9  to  12 
inches  high  and  are  spaced  about  24  inches  apart  each  way. 


6  BULLETIN   50,   HAWAII   EXPERIMENT   STATION. 

The  number  of  sweet-potato  cuttings  required  per  acre  is  indicated 
in  the  following  table: 

Number  of  sweet-potato  cuttings  required  per  acre  when  planted  at  different  distances. 


Distance 

apart  of 

rows. 

Distant    '  ^umber  of  ' 

auarthi    i  sweet-potato 

a^V      !     cuttings     : 

r0Wi>-           per  acre. 

Distance    j    Distance 

apart  of         apart  in 

rows.              rows. 

Number  of 

sweet-potato 

cuttings 

per  acre. 

Feet. 
2 

f 

3 

Feet. 
2 

10,  S90 
6,008 
9,680 
7,260    ; 

Feet. 
3 
3} 

4 

Feet. 
3 

? 

4,S40 
8,297 
5,445 

CULTIVATION. 


The  sweet  potato  requires  careful  cultivation  when  it  is  grown  in 
soil  other  than  sand  or  sandy  loam.  The  shape  and  size  of  the  roots 
are  materially  affected  by  the  physical  condition  of  the  soil.  Cultiva- 
tion loosens  and  aerates  the  soil,  and  puts  the  plant  food  in  such 
condition  that  it  can  readily  be  assimilated  by  the  crop.  It  also  keeps 
down  weed  growth  and  helps  to  conserve  soil  moisture. 

The  first  cultivation  should  be  given  when  the  vines  are  about 
8  inches  long,  or  sooner,  if  the  field  is  weedy.  Cultivation  consists 
in  turning  under  the  weeds  and  throwing  the  soil  from  the  furrow  to 
the  ridge  with  a  small  one-horse  plow,  supplemented  with  hoeings 
to  bring  the  soil  up  around  the  plant.  When  the  vines  are  grown 
under  level  cultivation,  a  harrow  should  be  used  on  the  soil.  Usually 
three  or  four  cultivations  are  sufficient  to  keep  the  soil  in  good  tilth 
until  the  field  is  overrun  with  vines.  Weed  growth  is  promoted  dur- 
ing the  rainy  season,  and  the  ground  then  requires  frequent  cultiva- 
tion. Sweet  potatoes  can  best  be  cultivated  with  a  hoe  when  they 
are  grown  on  small  areas. 

FERTILIZERS. 

The  sweet  potato  readily  responds  to  applications  of  fertilizer.  At 
the  experiment  station  an  increase  of  42  per  cent  in  yield  resulted 
from  the  application  of  a  mixture  of  sodium  nitrate,  potassium 
sulphate,  and  acid  phosphate,  in  the  proportions  of  75,  150,  and  400 
pounds,  respectively,  per  acre.  At  the  substation  at  Haiku,  Maui, 
sodium  nitrate,  potassium  sulphate,  and  equal  parts  of  reverted 
phosphate  and  acid  phosphate,  in  the  proportions  of  100,150,  and  250 
pounds,  respectively,  per  acre  gave  the  best  results. 

It  is  suggested  that  the  potassium  sulphate  be  increased  in  the 
first  formula  to  200  pounds  for  very  sandy  soil,  half  of  the  fertilizer 
being  applied  when  the  plants  have  been  set  for  about  five  weeks, 
and  the  rest  five  weeks  later.  The  fertilizer  should  be  uniformly 
applied.  The  Haiku  formula  is  recommended  for  use  on  areas  where 
the  soil  is  adapted  to  the  production  of  sweet  potatoes,  but  on  elevated 
regions  maturity  is  delayed  by  reason  of  the  altitude,  the  reverted 
phosphate  being  gradually  made  available  to  the  plants  during  the 
protracted  period  of  growth. 


Bui.  50,  Hawaii  Agr.  Expt.  Station. 


Plate 


Sweet  Potatoes  Produced  in   Heavy  Clay  Soil.    Irregular  in  Shape 
and  Unmarketable. 


Bui.  50,  Hawaii  Agr.  Expt.  Station. 


Plate  II. 


Fig.    I. -Vines  Turned  to  One  Side  to   Promote   Evaporation   from 
Soil  of  Excessive  Moisture  Caused  by  Standing  Water. 


Fig.  2— Sweet  Potatoes  Grown  by  Ridge  Method. 


THE    SWEET   POTATO    IX    HAWAII. 


IRRIGATION. 


Irrigation  is  not  an  important  consideration  in  connection  with 
sweet-potato  growing  in  Hawaii  owing  to  seasonal  conditions  and 
the  drought-resistant  character  of  the  plant.  It  thrives  and  pro- 
duces a  crop  of  roots  with  very  little  moisture.  The  type  of  soil  and 
the  amount  of  rainfall  largely  determine  where  and  when  the  crop 
can  be  grown  to  the  best  advantage.  When  rainfall  is  the  only 
source  of  moisture,  the  crop  should  be  frequently  cultivated  to  con- 
serve moisture  as  the  dry  season  approaches. 

The  plants  should  be  watered  sparingly  when  they  are  ^rown  on 
small  areas  or  in  localities  where  irrigation  is  possible.  It  is  impos- 
sible to  establish  a  definite  rule  for  time  of  irrigation,  because  some 
soils  retain  moisture  longer  than  do  others,  but  it  is  good  practice  to 
water  the  plants  when  the  soil,  to  a  depth  of  3  or  more  inches,  is 
comparatively  dry  to  the  touch. 

A  very  effective  way  of  irrigating  the  sweet-potato  crop  is  by 
turning  the  water  into  alternate  furrows.  This  practice  permits 
thorough  saturation  of  the  soil  immediately  surrounding  the  plant 
and  precludes  the  possibility  of  its  packing  later.  In  flat  culture 
light  irrigation,  rather  than  heavy,  should  be  practiced. 


PRUNING. 


Some  of  the  local  growers  cut  the  sweet-potato  tops  from  the 
cultivated  field  for  feeding  hogs.  To  determine  the  effect  of  such  a 
practice  upon  yield  of  roots  the  experiment  station,  in  1917,  carried 
on  a  test,  covering  eight  months,  with  the  Yellow  l  am  variety  of 
sweet  potato.  The  plants  were  set  in  12  rows,  each  100  feet  long, 
and  the  vines  were  cut  once  a  week  after  they  had  attained  a  length 
of  18  inches.     The  following  table  gives  the  result  of  the  test: 

Effect  on  yield  of  cutting  back  sweet-potato  vines. 


Rows. 


1,  2.  3,  4.... 
5,6,  7,8.... 
9,  10,  11,  12. 


Length  to 
vhich  vines 
were  cut. 


Inches. 

12 
C1) 

18 


Calculated 
acre  yield. 


Pounds. 
6, 859. 4 
21, 857. 6 
10, 779. 1 


1  Not  pruned. 

The  above  table  shows  that  the  yield  of  sweet  potatoes  is  greatly 
decreased  when  the  vines  are  cut  back.  The  rows,  the  vines  of  which 
were  kept  within  12  to  18  inches  long,  produced  a  yield  of  roots 
approximating  31  and  49  per  cent,  respectively,  of  a  crop  as  compared 
with  the  unpruned  vines.  The  test  showed  that  many  pruned  vines 
were  devoid  of  enlarged  marketable  roots.  Apparently  the  shorter 
the  vine  is  cut,  the  greater  will  be  the  reduction  in  yield. 

HARVESTING. 

The  period  of  maturity  of  the  sweet  potato  differs  with  the  variety 
and  the  locality  in  which  it  is  grown.  Some  varieties  mature  a  crop 
much  earlier  than  do  others.     In  all  varieties  the  period  required  for 

55378—23 2 


8  BULLETIN   50,   HAWAII  EXPERIMENT   STATION. 

maturity  is  lengthened  when  the  crop  is  grown  at  the  higher  eleva- 
tions. Sweet  potatoes  are  classified  in  three  groups,  those  maturing 
in  three  months,  or  early  varieties;  those  maturing  between  four 
and  five  months,  or  medium  early  varieties;  and  those  maturing 
between  six  and  seven  months,  or  late-maturing  varieties.  Some 
suppose  that  the  crop  is  mature  and  ready  for  harvesting  in  about 
four  or  five  months  after  planting  if  the  leaves  turn  yellow.  Yellow- 
ing of  the  leaves  is  not  always  an  indication  of  ripening  and  may  be 
caused  b}^  drought  or  the  disease  known  as  stem  rot.  Excessive 
rainfall,  on  the  other  hand,  stimulates  the  growth  of  the  vines  and 
causes  the  foliage  to  remain  green  for  months  after  the  roots  have 
matured.  Again,  it  is  supposed  that  the  crop  has  matured  if  the 
roots  remain  white  upon  being  broken,  and  that,  vice  versa,  immatur- 
ity is  shown  by  discoloration  of  the  broken  surface.  In  experiments 
with  sweet  potatoes  which  were  known  to  be  immature,  the  experi- 
ment station  found  no  discoloration  to  occur  after  breaking  except 
that  due  to  the  milky  substance,  latex,  which  is  also  present  in  the 
mature  root. 

On  account  of  its  excellent  keeping  qualities  the  sweet  potato  in 
Hawaii  may  be  left  unharvested  in  the  field  for  months  without  like- 
lihood of  its  decaying,  even  when  the  period  of  maturity  has  passed. 
If  the  roots  are  kept  in  the  ground  after  maturity,  not  only  will  the 
quality  be  improved  materially,  but  the  yield  will  be  considerably 
increased.  The  question  of  when  to  harvest,  then,  is  not  an  impor- 
tant one,  so  far  as  maturity  is  concerned.  Under  no  circumstances, 
however,  should  the  crop  be  harvested  after  a  rain  when  the  ground 
is  very  moist,  if  the  roots  are  intended  for  market  and  a  large  area 
is  to  be  harvested.  When  the  soil  is  wet,  sweet  potatoes  are  difficult 
to  harvest  and  the  earth  sticks  to  them. 

A  gardener  whose  sweet-potato  area  is  small  may  harvest  his 
crop  before  it  matures  fully.  To  harvest  the  required  quantity  of 
roots  without  disturbing  every  hill,  the  vine  should  be  carefully 
lifted  and  the  ground  examined  for  large  cracks  or  crevices  around 
the  plant.  These  cracks  are  found  when  the  soil  surface  is  dry  and 
occur  where  extra-sized  roots  are  growing. 

In  small  areas,  and  where  the  crop  is  grown  primarily  for  home 
consumption,  harvesting  may  be  facilitated  by  means  of  a  four- 
pronged  spading  fork.  After  the  vines  are  removed  the  fork  should 
be  inserted  in  the  soil  at  a  distance  of  8  or  10  inches  from  the  plant 
and  the  roots  lifted  and  brought  to  the  surface. 

When  the  crop  is  grown  on  a  large  scale  in  Hawaii,  a  turnplow  is 
used  for  harvesting  after  the  vines  have  been  removed  by  hand. 
The  sweet  potatoes  are  brought  to  the  surface  by  plowing,  thrown 
to  one  side  by  men,  and  later  are  graded  for  market.  The  roots 
should  be  left  in  the  field  and  exposed  to  the  sun  for  a  few  hours  to  dry 

YIELD. 

The  yield  of  sweet  potatoes  depends  largely  upon  the  locality  and 
soil  in  which  the  crop  is  grown,  the  kind  and  amount  of  fertilizer 
used,  the  culture  given,  and  finally  upon  the  variety  itself.  A  yield 
of  10  tons  per  acre  can  be  secured  from  a  crop  that  is  grown  under 
favorable  conditions.  As  high  a  yield  as  17£  tons  of  roots  and 
21£  tons  of  vines  per  acre  has  been  produced  at  the  Haiku  sub- 


THE    SWEET  POTATO    IX    HAWAII.  9 

station.  If  the  yield  is  estimated  on  a  small  basis,  100  plants, 
spaced  4  by  2  feet  apart,  should  produce  at  least  150  pounds  of 
merchantable  and  50  pounds  of  cull  roots. 

In  varietal  tests  covering  a  period  of  five  years  at  the  central  station 
at  Honolulu  an  average  yield  of  4  tons  per  acre  was  obtained  from 
all  the  varieties  tested.  In  these  tests  the  early  maturing  varieties 
were  less  productive  than  either  the  medium  or  late  varieties.  The 
following  table  gives  the  comparative  yield  of  a  number  of  these 
varieties : 

Comparative  yield  of  sweet  potatoes  tested  at  the  central  station. 


Variety. 

Yicld                                    Variety 
per  acre.                                 '  ane  •  ■ 

Yield 
per  acre. 

Delicious 

Tens. 

3.7      Native  Red 

4.1      ' "  Yellow  Yam''' 

Tons. 
6.0 

6.6 

3.  -      New  Era 

3. 1      Tantalus 

5.  i 

6.7 

No.  111-A 

6.2 

The  yield  of  sweet  potatoes  may  be  increased  by  proper  culture 
and  the  application  of  fertilizers,  or  by  the  selection  through  several 
generations  of  individual  hills  producing  heavy  crops  of  desirable 
and  well-shaped  roots.  The  most  opportune  time  to  select  for 
improvement  is  when  the  crop  is  being  harvested.  Hills  are  then 
dug  individually,  showing  numbers  of  roots  of  varying  shape  and 
size.  Vine  cuttings  should  be  taken  from  the  hills  which  contain 
the  largest  number  of  roots  of  good  size  and  uniform  shape  (PL  III, 
fig.  2).  These  should  be  carefully  labeled  and  notes  made  concerning 
their  individual  parents. 

Many  hills  should  be  selected  for  foundation  work,  because  some 
of  them  will  fail  to  transmit  their  prolific  characteristics.  Improve- 
ment should  then  be  continued  by  eliminating  the  unproductive 
vines  and  retaining  those  bearing  heavy  crops  of  roots.  (PI.  IV, 
fig.  1.)  Within  a  period  of  five  or  six  years,  representing  10  or 
more  generations,  the  grower  will  be  able  to  establish  a  prolific  strain 
of  fine  quality  which  he  is  entitled  to  call  his  own  and  to  give  a  special 
name  if  its  characters  are  sufficiently  distinct. 

GRADING. 

Although  sweet-potato  grading  has  been  advocated  in  Hawaii  for 
many  years,  wholesalers  continue  to  purchase  solely  on  the  basis 
of  exchange  of  so  much  money  for  so  much  weight,  regardless  of 
kind  of  material  weighed.  The  local  farmers  raising  diversified 
crops  recognize  the  importance  of  standardizing  agricultural  produce 
as  a  means  of  building  up  their  business,  but  the  local  growers,  who 
supply  the  markets  with  sweet  potatoes  for  culinary  purposes,  are 
indifferent  to  suggestions  regarding  grading,  either  because  the  planting 
of  this  crop  is  incidental  to  their  specialized  crops  and  the  area  is  so 
small  that  it  does  not  justify  the  extra  labor  expended  in  grading, 
or  they  have  learned  that  quotations  are  the  same  for  graded  and 
ungraded  sweet  potatoes.  Standardization  of  the  crop  will  not 
become  a  reality  until  uniformity  of  size,  shape,  and  color  enters  into 


10  BULLETIN   50,   HAWAII   EXPERIMENT    STATION. 

the  transaction  and  sweet  potatoes  are  marketed  practically  free 
fro:n  cuts,  bruises,  decay,  scars,  cracks,  and  other  defects  resulting 
from  carele'ss  handling,  as  well  as  from  diseases  and  insect  pests. 

SELECTION  OF  SWEET  POTATOES  FOR  HOME  USE. 

Fully  15  to  25  per  cent  of  the  total  weight,  depending  upon  the 
shape  of  the  roots,  is  lost  in  the  paring  of  sweet  potatoes  which  are 
intended  for  culinary  use.  In  other  words,  there  is  removed  in  the 
form  of  paring  from  H  to  2\  pounds  by  weight  for  every  10  pounds 
of  sweet  potatoes  pared.  For  this  reason  there  should  be  selected 
for  home  use  regular-shaped,  uniform,  smooth,  firm,  and  fresh- 
looking  roots  rather  than  shriveled  or  irregular-shaped  specimens. 

STORING. 

In  Hawaii  the  sweet  potato  is  not  stored  for  future  use  for  any 
great  length  of  time,  because  the  crop  can  be  harvested  during  any 
month  of  the  year.  After  sunning  for  a  few  hours  in  the  field  the 
roots  are  taken  to  a  shed  and  spread  out  in  shallow  piles  to  cure 
thoroughly.  A  number  of  sweet-potato  growers  on  Oahu  make  it  a 
practice  to  harvest  just  enough  to  supply  the  current  needs  of  the 
wholesaler,  and  the  native  Hawaiians  never  harvest  more  than  the 
actual  quantity  of  roots  needed  to  supply  the  household  for  the  day. 
In  this  they  show  an  appreciation  of  the  quality  of  the  freshly  dug 
root  and  a  knowledge  of  how  to  eliminate  the  problem  of  storage. 
The  small  gardener  would  do  well  to  adopt  their  practice. 

SHRINKAGE  IN  STORAGE. 

Data  obtained  at  the  experiment  station  on  shrinkage  occurriDg 
in  sweet  potatoes,  which  are  sacked  and  left  in  a  cool  but  dry  room, 
show  that  a  loss  of  12  per  cent  of  the  original  total  weight  takes 
place  in  21  days,  approximately  6  per  cent  of  which  is  lost  during  the 
seven  days  immediately  following  harvesting  and  6  per  cent  during 
the  next  14  days.  It  is  evident,  therefore,  that  sweet  potatoes  should 
be  carefully  stored  to  reduce  the  loss  by  shrinkage  to  a  minimum. 
Storage  houses  intended  for  sweet  potatoes  should  have  wooden 
roofing,  which  does  not  have  the  same  heat-retaining  properties  as 
does  galvanized-iron  roofing,  and  the  storage  rooms  should  be  kept 
cool  and  dark. 

COST  OF  PRODUCTION. 

The  cost  of  producing  a  crop  of  sweet  potatoes  depends  largely  upon 
the  locality  where  it  is  grown  and  the  method  of  growing.  On  level 
land,  where  labor-saving  devices  can  be  used  to  do  considerable  of 
the  work,  the  cost  per  acre  is  less  than  is  the  case  on  hilly  lands  where 
the  greater  part  of  the  work  has  to  be  done  by  manual  labor.  In 
Hawaii,  where  labor  is  rated  as  an  expensive  item,  the  cost  of  pro- 
ducing an  acre  of  sweet  potatoes  ranges  between  $80  and  SI 00, 
exclusive  of  the  charge  for  rental  of  land,  fertilizers,  and  cost  of 
sacks.  The  following  table  gives  the  comparative  cost  of  producing 
sweet  potatoes  at  the  central  station,  the  substation  at  Haiku,  and 
on  the  mainland: 


Bui.  50,  Hawaii  Agr.  Expt.  Station. 


Plate  II 


Fig.  I.— Sweet  Potatoes  Grown  by  Mound  or  Hill  Method. 


Fig.  2-Cluster  of  Sweet  Potatoes  of  Uniform  Size  and  Shape. 


Bui.  50,  Hawaii  Agr.  Expt.  Station. 


Plate  IV. 


Fig.   I.— Hills  Bearing  Sweet  Potatoes  in  Good  Clusters.    Cuttings 
Should  Be  Made  from  Plants  of  This  Type. 


Fig.  2.— Sweet  Potatoes  Badly  Damaged  by  Stem  Borers. 


THE   SWEET    POTATO    IX    HAWAII.  11 

Comparison  of  the  cost  of  producing  stbeet  potatoes  in  Hawaii  and  on  the  mainland.1 

Location. 


Expenditures. 

Central       Haiku  sub- 
station,        station. 


Mainland. 


Plowing  (3  times) ;         $18/00  ,  118.0  $15.00 

disking- 6.00  :  9.00   


ing 3.00 

Plank  dragging , 1. 00 

Rideins. . . 

Plants 

Planting... 
Cultivation 
Removing  vines 


6.00  5.00 

15.00  15.00 

8.00  6.50 

6.00  ,              4.50 


-11.00  2  9.00 

15.00 

10.00  ! 

Hauling |  5.00  |  7.50 


4.00 
10.00 
5.00 
5.00 


Harvesting 16.00  j  15.00  |  25.00 

Extra  cost,  due  to  experimental  data  obtained 10. 00 


104.00  ;  90.50  !  64.00 


1  See  Farmers'  Bui.  324,  Sweet  Potatoes,  p.  38. 

-  The  item  of  cost  in  removing  the  vines  is  recovered  by  disposing  of  the  vines  as  a  soiling  crop.    They 
were  delivered  to  the  piggery  of  the  Haiku  substation  for  S5  a  ton. 

The  above  table  shows  that  the  cost  of  production  at  the  experi- 
ment station  was  considerably  more  than  was  the  case  at  either  the 
Haiku  substation  or  on  the  mainland.  This  difference  was  largely 
due  to  the  great  care  which  was  exercised  in  the  planting,  cultivation, 
and  weighing  of  the  roots  and  vines,  grown  as  they  were  under  experi- 
mental conditions  rather  than  under  ordinary  field  conditions. 

The  figures  of  the  Haiku  substation  serve  as  representative  items 
in  the  cost  of  production  when  the  crop  is  grown  on  a  commercial 
basis  in  Hawaii.  At  this  substation  the  cost  per  cultivation  was 
SI. 50.  or  three  cultivations  for  84.50;  the  cost  of  removing  the  vines 
and  loading  them  on  to  the  wagon  was  at  the  rate  of  SI. 50  per  ton, 
or  6  tons  for  S9.  An  item  of  15  cents  represents  the  actual  cost  of 
preparing,  harvesting,  cleaning,  grading,  and  sacking  a  100-pound 
sack  of  sweet  potatoes  for  delivery  from  the  field.  On  the  basis  of 
100  sacks  per  acre  the  cost  would  be  $15,  as  indicated.  The  hauling 
charge  was  81.50  per  ton. 

The  mainland  cost  for  producing  an  acre  of  sweet  potatoes  is  decid- 
edly low,  as  shown  by  the  table.  The  use  of  labor-saving  devices  and 
the  growing  of  the  crop  in  extensive  areas  make  the  low  figures 
possible.  The  item  of  S15  for  three  plowings  also  includes  the  cost 
of  disking  and  leveling  the  surface  prior  to  ridging,  and  the  item  of  825 
for  harvesting,  includes  cost  of  cleaning,  grading,  sacking,  and 
hauling. 

INSECT  ENEMIES  AND  METHODS  OF  CONTROL. 

The  sweet  potato  in  Hawaii  is  attacked  by  a  number  of  insects 
which  feed  upon  the  leaves,  stems,  and  roots.  Leaf-eating  insects 
cause  little  permanent  injury  to  the  plant,   owing  to  its  vigorous 

frowth.  and  they  can  be  brought  under  control  with  proper  measures. 
;  Tactically  all  of  the  insects  attacking  the  crop  have  been  described 
in  Bulletin  22  of  this  station,  and  only  those  which  are  especially 
injurious  are  mentioned  here. 


12  BULLETIN    50,   HAWAII   EXPERIMENT   STATION. 

INSECTS  INJURING  THE  FOLIAGE. 

Japanese  Beetle  (Adoretus  tenuimaculatus). 

The  Japanese  beetle  perforates  the  foliage  of  the  sweet-potato 
plant.  The  pest  can  be  brought  under  control  by  spraying  the  under 
side  of  the  foliage  with  a  mixture  containing  1  pound  of  lead  arsenate 
in  20  gallons  of  water. 

Sweet-Potato  Leaf-Miner  (Bedellia  orckilella). 

The  leaf-miner,  after  the  stem  borer,  is  perhaps  the  most  destructive 
insect  enemy  of  the  sweet-potato  vine.  Its  injuries  are  confined  to 
the  foliage,  which  as  a  result  presents  a  notched  and  withered  appear- 
ance and  bears  traces  of  excrement.  The  newly  hatched  ,  larvae 
penetrate  the  leaves  and  feed  upon  the  green  coloring  matter.  The 
leaves  gradually  lose  their  color  and  wither,  and  the  leafstalks  drop. 

The  leaf-miner  can  best  be  controlled  by  the  removal  and  burning 
of  severely  infested  foliage  and  by  the  practice  of  clean  cultivation. 

Sweet-Potato  Sphinx  (Rerse  convolvuli). 

During  the  rainy  season  the  sphinx  caterpillar  periodically  appears 
on  sweet-potato  vines.  The  eggs  are  usually  laid  on  the  under  side 
of  the  leaves  of  the  wild  morning-glory  Ipomcea  digitata,  and  occasion- 
ally on  those  of  the  sweet  potato.  The  caterpillar  is  from  2  to  6  inches 
long,  ranges  from  light  green  to  dark  brown  in  color,  and  is  charac- 
terized by  a  single  black  horn. 

As  a  means  of  control,  all  host  plants,  such  as  morning-glory  and 
pigweed  {Portulaca  oleracea),  should  be  destroyed.  On  vacant  lots 
in  the  Waialae  district  of  Oahu  wild  morning-glory  vines  should  be 
destroyed  to  protect  the  surrounding  fields  of  sweet  potatoes  from 
attack  by  the  sphinx  caterpillar. 

Spraying  the  under  side  of  the  leaves  with  arsenate  of  lead  solution 
(1  pound  of  the  arsenate  to  20  gallons  of  water)  will  materially  reduce 
the  number  of  caterpillars.  This  pest  may  also  be  kept  in  check  by 
picking  the  larvae  from  the  vines  growing  on  small  areas. 

Sweet-Potato  Leaf-Roller  (Phlyctxnia  despecta). 

This  leaf-roller  is  one  of  the  minor  insect  pests  attacking  the  foliage 
of  the  sweet-potato  crop.  In  extreme  cases  it  skeletonizes  the  leaves 
and  envelopes  the  remaining  structure  in  a  filmy  web.  The  young 
caterpillar,  although  colorless,  usually  appears  green,  owing  to  the 
green  contents  of  the  alimentary  tract,  which  can  be  seen  through 
the  body.     Its  hiding  place  is  on  the  under  side  of  the  leaves. 

An  application  of  lead  arsenate  to  the  leaves  is  recommended  as  a 
control  measure.     Clean  cultivation  should  also  be  practiced. 

INSECTS  INJURING  THE  STEM. 

Cutworm  (Agrotis  ypsilon). 

Cutworms  attack  the  sweet-potato  vine  at  night  during  the  early 
growing  period,  eating  the  terminal  and  axillary  buds  and  sometimes 
completely  cutting  oil  the  growing  vine  at  the  base. 

As  a  control  measure  the  use  of  poison  bait,  composed  of  1  pound 
of  molasses,  J  pound  of  lead  arsenate,  and  10  pounds  of  bran, 
thoroughly  mixed  and  spread  near  the  base  of  the  vines,  is  recom- 


THE    SWEET  POTATO   IN    HAWAII.  13 

mended.  If  the  soil  around  the  base  of  the  plant  is  stirred  with  a 
small  stick,  the  worms  will  leave  their  hiding  places  and  can  then  he 
easily  destroyed. 

INSECTS  INJURING  THE  ROOTS. 

Sweet-Potato  Weevils  (Cryptorhynchus  batatse  and  Cylcs  formicarius) . 

Sweet-potato  weevils  are  quite  destructive  at  times,  thoroughly 
channeling  the  roots  with  larval  burrows  and  causing  the  vines  to 
shrivel  and  decay. 

As  a  control  measure,  all  infested  potatoes  and  trash  above  ground 
should  be  burned  and  clean  cultivation  should  be  practiced. 

INSECTS  INJURING  BOTH  STEM  AND  ROOT. 

Stem  Borer  {Omphisa  anastomosalis). 

The  stem  borer  is  perhaps  the  most  destructive  of  all  the  insects 
attacking  the  sweet  potato  in  Hawaii.  It  not  only  burrows  large  and 
distinct  tunnels  in  the  stems  of  the  plant,  but  also  damages  the  root 
crop.  (PI.  IV,  fig.  2.)  In  a  severely  infested  field  the  vines  die  and 
the  yield  of  roots  is  greatly  reduced. 

Stomach  poisons  are  of  little  avail  in  connection  with  the  stem 
borer,  as  most  of  its  life  is  passed  within  the  stem  or  the  roots.  Prob- 
ably the  most  effective  way  of  combating  it  is  by  the  practice  of  clean 
cultivation.  All  infected  vines  and  roots  should  be  gathered  and 
burned.  In  severely  infested  small  areas  the  borers  can  be  crushed 
within  the  stem  by  the  pressure  of  the  ringers  up  and  down  the 
tunneled  vines. 

FUNGUS  DISEASES  AND  MEANS  OF  CONTROLLING  THEM. 

The  sweet-potato  plant  is  attacked  by  a  number  of  diseases  which 
are  divided  into  two  classes:  (1)  Those  attacking  the  crop  in  the 
field,  and  (2)  those  attacking  the  crop  in  storage. 

DISEASES  ATTACKING  THE  CROP  IN  THE  FIELD. 

Black  Rot  {Sphaeronema  fimbriatum). 

Black  rot,  due  to  the  fungus  S.  fimbriatum,  is  not  confined  to  the 
field,  but  also  attacks  the  crop  in  storage.  Infected  roots,  when 
taken  from  the  field,  show  discoloration  in  small  patches  which  enlarge 
and  finally  affect  the  whole  root.  Cooked  sweet  potatoes  which  are 
affected  with  black  rot  have  a  bitter  taste. 

Stem  Rot  (Fusarium  batatatis). 

Stem  rot,  caused  by  an  organism  (F.  batatatis),  commonly  attacks 
the  plant  and  its  roots.  As  the  name  indicates,  the  stem  is  affected, 
usually  becoming  blackened  in  the  advanced  stage,  and  the  foliage 
turns  a  dull  yellow  and  drops.  As  a  rule  the  vine  wilts,  and  in  the 
instances  where  it  does  survive  the  roots  are  discolored. 

Scurf  (Monilochsetes  infuscans). 

Scurf,  also  known  as  soil  stain,  is  due  to  a  fungus  (i¥.  infuscans) 
which  adheres  in  such  a  way  to  the  roots  as  to  be  mistaken  for  soil 
particles.     It  is  not  a  serious  disease  and  damages  the  potato  only  by 


14  BULLETIN    50,   HAWAII   EXPERIMENT   STATION. 

discoloring  the  skin.  A  white-skinned  variety  when  attacked 
presents  a  dark-gray  appearance,  while  a  red-skinned  variety  turns 
black. 

Foot  Rot  (Plenodomus  destruens). 

Foot  rot  is  indicated  by  rotting  of  the  stem  near  the  surface  of  the 
soil.     The  infected  plant  presents  a  rather  wilted  appearance. 

Blight  or  Wilt  (Sclerotium  rolfsii). 

Blight  or  wilt  is  indicated  by  decay  of  the  plant  at  the  base  and  by 
moldy  white  growth. 

Texas  Root  Rot  {Ozonium  omnivorum). 

Texas  root  rot  is  indicated  by  a  spotting  of  the  roots  and  a  wilting 
of  the  vine. 

Pit  or  Pox  (Cytospora  batatas). 

Pit  or  pox,  also  known  as  soil  rot,  is  indicated  by  malformation 
and  girdling  of  the  roots  of  the  plants,  accompanied  by  low  yield. 

DISEASES  ATTACKING  THE  CROP  IN  STORAGE. 

Soft  Rot  and  Ring  Rot  (Rhizopus  nigricans). 

Soft  rot  is  indicated  by  decay  of  the  root  at  one  end,  the  diseased 
part  becoming  very  soft. 

Ring  rot  is  indicated  by  a  softening  of  the  roots  between  the  ends. 

Dry  Rot  (Diaporthe  batatatis). 

Dry  rot  is  indicated  by  shriveling  of  the  roots,  unaccompanied  by 
softness. 

Java  Root  Rot  {Diplodia  tubericolu). 

Java  root  rot  is  indicated  by  blackened  flesh. 

LEAF  DISEASES. 

Three  minor  diseases  which  affect  the  foliage  of  the  plant  are  leaf- 
blight,  giving  the  plant  a  withered  appearance;  leaf -spot,  identified 
by  minute  white  specks  on  the  leaves;  and  white-rust,  indicated  by 
small  white  spots.  Being  only  minor  diseases,  they  require  no  treat- 
ment when  occurring  on  a  large  area.  Infected  leaves  should  be 
removed  from  small  areas,  however,  and  burned. 

CONTROL. 

It  is  only  by  the  most  careful  management  that  sweet-potato 
diseases  can  be  eliminated  from  the  field.  The  selection  of  prop- 
agating materials  from  absolutely  sound  plants,  the  careful  examina- 
tion of  the  root  crops  and  vines  before  moving  from  one  field  to 
another,  and  the  practice  of  crop  rotation  are  the  only  effective 
measures. 

Cuttings  which  are  intended  for  propagation  should  be  selected 
with  care  if  they  are  taken  from  infected  areas.  They  should  be 
treated  with  a  solution  of  bichlorid  of  mercury  (1  ounce  of  bichlorid 
of  mercury  to  8  gallons  of  water) ,  being  submerged  in  the  solution  for 
five  minutes.  Vines  or  roots  showing  signs  of  disease  should  be 
removed  from  the  field  and  burned.     If  a  locality  continues  to  be 


THE    SWEET  POTATO    IN    HAWAII. 


15 


infected,  notwithstanding  its  receiving  every  precaution  to  make  it 
disease  free,  it  should  be  allowed  to  lie  fallow  for  a  year  or  two  and 
then  be  planted  with  about  20  varieties  of  sweet  potatoes.  Selection 
should  then  be  made  for  future  planting  from  the  varieties  appearing 
healthy  and  producing  luxuriant  foliage  and  good  crops  of  sound 

roots. 

VARIETIES  OF  SWEET  POTATOES  IN  HAWAII. 

Approximately  70  distinct  varieties  of  sweet  potatoes  having 
Hawaiian  names  are  known  to  the  native  growers,  and  fully  200 
others  are  either  unnamed  or  bear  English  names.4  Many  of  these 
varieties  are  undoubtedly  cultivated  elsewhere  under  different  names. 
For  practical  and  commercial  purposes  the  following  listed  varieties 
are  recommended  for  table  use  and  for  feeding  to  live  stock : 

Varieties  of  sweet  potatoes  which  are  adapted  for  both  human  consumption  and  for  live- 
stock feeding.1 


Varieties. 

Color  of  skin. 

Color  of  flesh. 

Early  maturing: 

Delicious  Yellow 

Purplish  pink 

Pumpkin. 
Orange. 
Light  yellow. 

White. 

do 

Medium  earlv  maturing: 

No.  111-A 

White 

do 

Light  yellow. 
White. 

Native  Red 

Red 

Straw 

Pumpkin. 

Light  cream. 
Cream. 
Light  yellow. 

Late  maturing: 

Dark  buff 

Tantalus 

Light  brown 

White 

1  The  late-maturing  varieties  are  especially  recommended  for  live-stock  feeding  because  they  are  prolific 
bearers,  and  good  keepers,  and  contain  a  high  percentage  of  starch. 

A  great  variety  of  potatoes  is  needed  in  Hawaii  to  meet  the  demands 
of  the  cosmopolitan  population.  The  occidental  population  in 
Hawaii  prefers  sweet  potatoes  having  orange  or  pumpkin-colored 
flesh,  while  the  oriental  population  prefers  a  red-skinned  variety 
having  flesh  varying  from  white  to  canary  in  color. 

COMPOSITION  OF  THE  SWEET  POTATO. 

Sweet  potatoes  vary  considerably  in  chemical  composition  ac- 
cording to  the  variety  and  the  place  where  they  are  grown.  The 
followring  table  gives  a  comparison  of  the  sweet  potato  with  other 
starchy  crops: 

Relative  value  of  the  sweet  potato  and  other  starchy  root  crops.a 


Crop. 


Sweet  potato  (whole). 
Sweet  potato  (peeled). 
Sweet  potato  (peeling) 

Cassava  (whole) 

Taro  (whole) 

Potato  (whole)  b 


Water. 

Ash. 

Crude 
protein. 

Carbo- 
hydrates. 

Per  cent. 

Per  cent. 

Percent. 

Per  cent. 

68.89 

0.90 

2.12 

27.83 

68.50 

.94 

2.18 

28.18 

74. 35 

.31 

1.29 

21.96 

64.17 

.86 

.77 

32.61 

60.55 

.73 

1.10 

37.49 

78.30 

1.00 

2.20 

18.50 

Fat. 


Per  cent. 
0.25 
.20 
1.03 
1.59 
.13 
.13 


a  Practically  compiled  from  Hawaii  Sta.  Press  Bui.  53. 
t>  Unpublished  analysis  made  by  the  Hawaii  Experiment  Station. 

*  Sweet  -potato  breeding  has  been  carried  on  by  the  experiment  station  since  1917,  since  which  time  mora 
than  700  seedlings  have  been  produced. 


16  BULLETIN   50,   HAWAII   EXPERIMENT  STATION. 

From  the  above  table  it  will  be  seen  that  the  sweet-potato  root 
compares  favorably  in  chemical  composition  with  the  three  other 
root  crops.  The  whole  sweet  potato  shows  a  higher  ash  and  crude* 
protein  content  and  a  lower  carbohydrate  content  than  does  either 
the  cassava  or  the  taro.  In  fat  content  it  is  lower  than  the  cassava, 
but  higher  than  the  taro.  A  comparison  of  the  sweet  potato  with 
the  potato  brings  out  the  significant  fact  that  the  former  is  about 
as  high  in  crude  protein  content  as  is  the  latter  and  that  it  is  much 
higher  in  carbohydrates  and  fat.  Notwithstanding  these  facts,  the 
potato  commands  the  higher  price  of  the  two  on  the  local  markets 
throughout  the  year. 

RECIPES. 

Sweet  potatoes  are  prepared  for  table  use  by  practically  all  of  the 
many  nationalities  in  Hawaii,  in  the  occidental  homes  and  hotels 
being  baked,  boiled,  or  braised,  and  in  the  oriental  homes,  boiled 
whole,  or  pared,  sliced,  and  then  boiled.  Starch  is  extracted  from 
the  raw  potato  for  the  preparation  of  a  paste-forming  meal  which 
can  be  fed  to  infants  and  also  to  adults.  Some  of  the  methods  of 
using  the  sweet  potato  are  given  below. 

SWEET-POTATO  BREAD. 

(1  loaf.) 


1  cupful  of  mashed  sweet  potatoes. 
1  teaspoonful  of  salt. 
1  tablespoonful  of  sirup,  if  desired 
4  tablespoonfuls  of  lukewarm  water. 


2\  cupfuls  or  more  of  sifted  flour. 

\  to  \  cake  of  yeast  (dry  or  compressed), 

or  from  2  to  4  tablespoonfuls  of  liquid 

yeast.5 


Use  left-over  boiled  or  baked  sweet  potatoes  or  boil  sweet  potatoes  in  their  jackets 
until  tender.  Pare  and  mash  the  sweet  potatoes  or  put  them  through  a  colander  or 
ricer  to  free  them  from  lumps. 

Short  process. — To  1  cupful  of  the  cool  mashed  sweet  potatoes  add  1  teaspoonful 
of  salt,  1  tablespoonful  of  sirup,  and  ^  cake  of  compressed  yeast  mixed  with  4  table- 
spoonfuls of  lukewarm  water,  or  4  tablespoonfuls  of  liquid  yeast.  It  may  be  necessary 
to  add  more  water  if  the  sweet  potatoes  are  rather  dry  or  mealy.  Add  to  this  h  to  1 
cupful  of  sifted  flour  and  stir  until  the  whole  is  thoroughly  mixed.  Cover  and  allow 
the  mixture  to  rise  for  about  two  hours  until  it  becomes  soft  and  light.  Knead  in 
another  quantity  of  flour  sufficient  to  make  a  dough  somewhat  stiff er  than  for  white 
bread.  Knead  the  mixture  until  it  is  smooth  and  elastic,  then  cover  and  let  it  rise 
again  until  it  becomes  very  light.  Knead,  mold,  and  finish  as  usual.  Allow  the  mass 
to  rise  in  the  pan  until  it  reaches  2\  or  3  times  its  original  bulk.  Bake  slowly  in  a 
moderately  hot  oven  for  at  least  50  minutes. 

Long  process. — To  1  cupful  of  the  cool  mashed  sweet  potatoes  add  1  teaspoonful  of 
salt  and  either  £  cake  of  yeast,  dry  or  compressed,  mixed  with  4  tablespoonfuls  of 
lukewarm  water,  or  2  tablespoonfuls  of  liquid  yeast.  Add  to  this  \  to  1  cupful  of 
sifted  flour.  Cover  and  set  to  rise  where  the  temperature  ranges  from  60°  to  70°  F. 
When  the  mixture  is  light  and  soft  add  the  sirup  and  knead  in  another  quantity  of 
flour  sufficient  to  form  a  smooth,  elastic,  and  rather  stiff  dough.  Cover  and  let  rise  a 
second  time  until  it  becomes  very  light.  Then  knead,  mold,  and  finish  as  directed 
for  the  short  process. 

Cooked  or  baked  squash,  pumpkin,  peas,  beans,  or  dasheen  may  be  substituted  for 
sweet  potatoes.  When  larger  quantities  of  sweet  potatoes  are  to  be  used,  less  water 
will  be  required;  that  is,  for  every  If  cupfuls  of  mashed  sweet  potatoes  only  2  table- 
spoonfuls of  water  per  loaf  are  needed.     In  this  case  less  flour  will  be  required. 


SWEET-POTATO  BISCUIT. 


2  cupfuls  of  sifted  flour. 
1  tablespoonful  of  salt. 

3  teaspoonfuls  of  baking  powder. 


I  cupful  of  mashed  sweet  potatoes. 
3  tablespoonfuls  of  shortening. 
Sufficient  water  or  milk  to  mix. 


» Liquid  yeast,  when  used,  should  be  included  in  the  total  liquid. 


THE    SWEET  POTATO   IN    HAWAII.  17 

Sift  the  flour,  salt,  and  baking  powder  together.  Cut  or  rub  the  cold  shortening 
into  this  mixture.  In  the  same  way  rub  into  this  flour  mixture  the  mashed  potatoes. 
Finally,  add  just  enough  cold  liquid  to  make  the  mass  cling  together.  Do  not  knead. 
Place  mass  on  floured  board,  roll  until  £  inch  thick,  and  cut  with  a  biscuit  cutter. 
Place  in  lightly  floured  biscuit  tins  and  bake  for  15  or  20  minutes  in  a  moderately 
hot  oven.     Bake  potato  breads  more  slowly  than  all-flour  breads. 

SWEET-POTATO  GREENS. 

The  tops  of  the  sweet  potato  vine  make  excellent  greens  for  the  table,  comparing 
in  this  respect  with  spinach.  Remove  the  tips  to  a  length  of  3  inches,  wash,  and 
place  in  saucepan  containing  water  and  salt.  Boil,  drain,  and  season  with  salt  and 
pepper  before  serving. 

SWEET-POTATO  PORRIDGE. 

(Oriental  method.) 

1  tablespoonful  of  sweet-potato  starch.        I  About  1  cupful  of  boiling  water. 
\  cupful  of  cold  water.  I  Sugar  to  sweeten. 

Place  the  sweet-potato  starch  in  a  quart  container,  add  water,  and  mix  thoroughly. 
Gradually  add  boiling  water  and  stir  until  a  thick,  paste-like  porridge  is  formed.  Add 
sugar  to  sweeten  and  serve. 

SWEET-POTATO  SOUP. 

(Oriental  method.) 

Pare  sweet  potato,  cut  into  slices  about  one-fourth  inch  thick,  and  place  in  a 
saucepan  containing  barely  enough  water  to  cover.  Cook  over  a  moderate  fire  for  1£ 
hours.     A  little  sugar  may  be  added  if  desired. 

HAM  SMOTHERED  IN  SWEET  POTATOES.* 


1  slice  of  smoked  ham  cut  into  sizes  for 

serving. 
3  cups  of  raw  sliced  sweet  potatoes. 


2  tablespoonfuls  of  sugar. 

1  tablespoonful  of  butter  or  ham  fryings. 

1  cupful  of  hot  water. 


Broil  the  pieces  of  ham  lightly  on  both  sides  and  arrange  them  to  cover  the  bottom 
of  the  baking  dish.  Spread  the  slices  of  sweet  potato  over  them,  sprinkle  with  sugar. 
Add  the  hot  water  and  extra  fat.  Cover  the  dish  and  bake  slowly  until  the  ham  is 
tender,  basting  the  potatoes  occasionally  with  the  gravy.     Brown  the  top  well. 

SWEET  POTATOES  AND  PEANUTS. 

1  quart  of  mashed  sweet  potatoes.  I  \  teaspoonful  of  cinnamon. 

2  tablespoonfuls  of  butter  or  other  fat.         I  1  teaspoonful  of  salt. 
1  cupful  of  roasted  peanuts  chopped  fine.  | 

Mix  the  ingredients  well;  form  into  a  mound  upon  a  shallow  baking  dish;  press  a 
tablespoonful  of  butter  into  the  top.     Heat  in  a  moderate  oven  until  brown. 

SWEET-POTATO  NUTS.* 

To  1  pint  of  boiled  and  mashed  potatoes  add  1  pint  of  toasted  bread  crumbs  rolled 
fine,  1  pint  mixed  nut  meats  chopped  fine  (peanuts  are  excellent);  season  with  salt, 
a  little  pepper,  also  sage  and  mace,  if  desired;  to  the  yolks  of  2  eggs  add  2  teaspoons  of 
baking  powder  and  whip  until  light;  pour  the  egg  mixture  into  the  first-mentioned 
mixture  and  stir  well;  form  into  small  cakes;  dip  each  into  the  whites  of  the  eggs,  then 
into  shredded  coconut,  and  brown  in  a  frying  pan  containing  a  little  pork  fat  (not  deep 
fat);  turn;  brown  on  both  sides. 

8  This  and  the  following  recipe  were  obtained  from  miscellaneous  sources  through  the  courtesy  of  the 
Bureau  of  Home  Economics  of  the  United  States  Department  of  Agriculture. 

7  This  and  the  following  recipes  were  obtained  from  the  Tuskegee  Normal  and  Industrial  Institute, 
Tuskegee  Institute,  Ala.,  through  the  courtesy  of  Dr.  G.  W.  Carver.  (Bui.  38,  How  the  Farmer  Can  Save 
His  Sweet  Potatoes.) 


18  BULLETIN   50,   HAWAII   EXPERIMENT   STATION. 

SWEET  POTATOES  WITH  ROAST  PORK. 

Parboil  the  desired  number  of  potatoes  with  the  skin  on  until  nearly  done;  remove 
and  skin;  put  in  the  baking  dish  with  the  nearly  done  roast;  cook  until  done,  and  serve 
with  the  pork. 

CHIPS. 

Cut  in  thin  slices,  steam  until  nearly  done,  allow  the  surplus  water  to  drain  off,  or 
dry  between  napkins,  fry  in  deep  fat  to  a  light  brown.     A  little  salt  adds  to  its  flavor. 

SWEET  POTATOES  BAKED  WITH  APPLES. 

Wash,  pare,  and  cut  4  medium-sized  sweet  potatoes  into  slices  about  \  inch  thick, 
pare  and  slice  4  apples  in  the  same  way;  put  in  baking  dish  in  alternate  layers;  sprinkle 
1£  cups  of  sugar  over  the  top,  scatter  \  cup  of  butter  in  lumps  over  the  top;  add  |  pint 
of  hot  water;  bake  slowly  for  1  hour;  serve  steaming  hot. 

SWEET-POTATO  PUFFERS. 

Whip  2  eggs  until  quite  light;  2  cupfuls  of  cold  mashed  potatoes;  1  cupful  of  flour 
into  which  1  teaspoonful  of  baking  powder  has  been  sifted.  The  potatoes  and  eggs 
should  be  worked  together,  then  the  flour  and  baking  powder;  roll  lightly;  cut  quickly, 
and  fry  in  deep  fat  like  doughnuts.     Some  think  a  little  spice  improves  the  flavor. 

SWEET-POTATO  PIE.s 

Boil  in  skins;  when  tender  remove  skins,  mash  and  beat  until  light;  to  each  pint  add 
a  pint  of  milk  and  4  eggs.     Season  and  bake  as  pumpkin  pie. 

SWEET-POTATO  GLACfi. 

Cut  in  slices  \  inch  thick,  wash,  and  place  in  deep  saucepan  spread  with  butter, 
season  with  a  little  grated  nutmeg  and  salt;  moisten  with  broth  or  water,  cover  and  let 
simmer  over  slow  fire  for  J  hour,  turning  the  slices  so  that  they  may  glaze  on  both  sides. 
Serve  with  drawn  butter  or  other  sauce. 

SWEET  POTATOES  STUFFED. 

Bake;  then  cut  off  one  end  and  scoop  out  the  inside;  season  with  butter,  pepper,  and 
salt;  beat  until  light;  replace  in  the  skin;  close  with  the  piece  cut  off  and  put  into  the 
oven  to  heat  through.     Serve  in  napldns.     Suitable  for  luncheon. 

A  SOUTHERN  DISH. 

Cut  cold  baked  sweet  potatoes  into  slices  and  put  into  an  earthern  dish;  add  sugar 
and  butter  to  each  layer  and  bake  until  slightly  browned. 

SWEET-POTATO  CROQUETTES. 

Take  2  cupfuls  of  mashed,  boiled,  steamed,  or  baked  sweet  potatoes;  add  the  beaten 
yolks  of  2  eggs  and  season  to  taste;  stir  over  the  fire  until  the  mass  parts  from  the  sides 
of  the  pan.  When  cold  form  into  small  croquettes,  roll  in  egg  and  bread  crumbs,  and 
fry  in  hot  lard  to  an  amber  color.  Serve  on  napkins.  The  croquette  mixture  may  be 
made  into  balls  inclosing  minced  meat.     When  used  in  this  way,  serve  with  sauce. 

SWEET-POTATO  PUREE. 

Mash  boiled,  steamed,  or  baked  sweet  potatoes,  season,  and  add  enough  hot  milk  to 
moisten;  serve  like  mashed  white  potato;  or  put  in  pudding  dish,  dress  the  top  with 
egg,  and  brown  in  the  oven.     Serve  with  sauce. 

SWEET  POTATOES  BAKED. 

Bake  like  potatoes,  without  breaking  the  skin.  When  done,  break  the  skin  in  one 
place  in  the  form  of  a  cross,  forcing  the  meat  partly  out,  cap  with  butter,  and  serve. 

»  This  and  the  following  recipes  have  been  taken  from  United  States  Department  of  Agriculture  Farmers' 
Bui.  129,  Sweet  Potatoes. 


THE    SWEET   POTATO    IX    HAWAII. 


19 


SWEET  POTATOES  AS  FEED  FOR  FARM  ANIMALS. 

When  the  sweet  potato  is  grown  on  a  large  scale  in  rotation  with 
other  crops  and  it  is  desired  to  economize  on  labor  in  harvesting  the 
roots,  cattle  and  sheep  may  be  pastured  on  the  area  to  consume  the 
vines,  and  later  hogs  may  be  turned  in  to  harvest  the  roots  for  them- 
selves. If  the  area  in  sweet  potatoes  is  exceptionally  large,  and  the 
above  system  of  harvesting  is  practiced,  the  grazing  area  should  be 
inclosed  with  a  portable  fence  to  prevent  waste  when  the  hogs  are 
turned  in  to  root.  Potatoes  which  are  brought  to  the  surface  but 
not  eaten  should  be  daily  gathered  from  the  paddocked  area  and  fed 
to  other  hogs. 

Sweet  potatoes  can  be  fed  to  horses  and  mules  as  a  supplement  to 
the  regular  carbohydrate  feed.  The  roots  should  be  cut  into  pieces 
with  a  vegetable  cutter  and  then  mixed  with  a  small  quantity  of  mo- 
lasses, so  that  the  animals  will  become  accustomed  to  them.  The 
work  mules  at  the  experiment  station  are  given  rolled  barley  in  the 
morniDg.  corn  at  noon,  and  chopped  sweet  potatoes  in  the  evening, 
when  corn  and  sweet  potatoes  are  available,  This  ration  keeps  them 
in  excellent  condition. 

Sweet  potato  tops. — In  the  hog-raising  and  dairy  enterprises  in 
Hawaii  large  quantities  of  the  succulent  garden  pigweed  or  purslane 
(Portulaca  ohracea)  and  honohono  (Comrndina  nudifora)  are  fed  to 
hogs  and  cows.  The  following  table  gives  the  chemical  composition 
of  sweet-potato  vines,  pigweed,  and  honohono: 

Composition  of  sweet-potato  vines,  pigweed,  and  honohono. 


Crop. 

Water. 

Ash. 

Crude    j    Carbo- 
protein.  ,  hydrates. 

Fat. 

Sweet  potato  vines. . 

Per  cent. 

87. 67 

95. 20 

Per  cent. 

1.27 

.96 

1.40 

Per  cent.  :  Per  cent. 
2.93            7.77 
1.04  1          2.71 
1.27  ,          7.58 

Percent. 

0.38 

.09 

Honchom 

89. 43 

.32 

SUMMARY. 

The  sweet  potato  belongs  to  the  morning-glory  family,  and  many 
varieties  bloom  profusely  in  Hawaii  from  November  to  April. 

In  ancient  times  the  crop  was  cultivated,  the  native  agriculturists 
evidently  appreciating  the  importance  of  selection  and  the  value 
of  alternating  the  crop  with  other  crops  to  improve  the  physical 
condition  of  the  soil. 

The  crop  needs  moderate  rainfall,  an  abundance  of  sunshine,  and 
warm  nights  for  best  development.  It  should  be  grown  in  a  well- 
drained,  moderately  fertile,  loose  sandy  soil.  It  responds  to  favor- 
able  treatment,  making  good  growth  and  producing  roots  of  fine 
quality  on  well-prepared  land  that  has  been  planted  with  legumes 
the  year  preceding.  The  depth  of  plowing  usually  practiced  for  corn 
is  satisfactory  for  sweet  potatoes. 

Cultivation  should  begin  when  the  vines  are  about  8  inches  long. 
Usually  from  three  to  four  cultivations  are  sufficient  to  keep  the  soil 
in  good  tilth  until  the  field  is  overrun  with  vines. 

The  plant  is  drought-resistant  and  produces  a  crop  of  roots  with 
very  little  moisture. 


20  BULLETIN   50,   HAWAII   EXPERIMENT   STATION. 

In  a  test  made  to  determine  the  effect  of  pruning  on  yield,  it  was 
found  that  production  was  considerably  decreased  when  the  vines 
were  cut  back.  Apparently  the  shorter  the  vine  is  cut,  the  greater 
will  be  the  reduction  in  yield  of  roots.  Yield  is  largely  influenced 
by  the  locality  and  the  soil  where  the  crop  is  grown,  the  kind  and 
amount  of  fertilizer  used,  the  cultivation  given,  and  finally  by  the 
variety  itself.  It  may  be  increased  by  proper  cultivation  and  the 
application  of  fertilizers,  or  by  the  selection  through  several  genera- 
tions of  individual  hills  producing  heavy  crops  of  well-shaped  roots. 

The  period  of  harvesting  differs  with  the  different  varieties.  Yel- 
lowing of  the  leaves  is  not  always  an  indication  of  ripening  and  may 
be  due  to  the  disease  known  as  stem-end  rot. 

Unfortunately  the  sweet  potato  is  not  graded  in  Hawaii.  Stand- 
ardization will  hardly  become  a  reality  until  uniformity  of  size, 
shape,  and  color  is  taken  into  consideration,  and  the  roots  are  mar- 
keted free  from  defects  resulting  from  careless  handling,  diseases, 
and  insect  pests.  Regular-shaped,  uniform,  smooth,  firm,  and 
fresh-looking  potatoes  should  be  selected  for  home  use. 

The  problem  of  storage  can  be  eliminated  by  harvesting  just 
enough  potatoes  to  meet  the  needs  of  the  wholesaler  or  the  house- 
holder. When  they  are  to  be  held  for  some  time,  sweet  potatoes 
should  be  carefully  stored  to  reduce  to  a  minimum  the  loss  by 
shrinkage. 

The  sweet-potato  plant  is  attacked  by  a  number  of  insect  pests 
and  fungus  diseases  which  can  be  brought  under  control  by  the  use 
of  proper  measures. 

In  chemical  composition  the  sweet  potato  compares  very  favor- 
ably with  taro,  cassava,  and  potato. 

On  account  of  their  high  feeding  value,  sweet-potato  vines  should 
be  used  as  feed  for  farm  animals.  They  are  greatly  relished  by  hogs 
and  dairy  cattle,  and  when  it  is  desired  to  practice  economy  of  labor 
in  harvesting,  the  former  may  be  turned  in  to  harvest  the  roots, 
after  the  latter  have  been  allowed  to  pasture  the  vines. 

o 


